As shown in FIG. 1, a grating 9 is mounted on an upper end segment of a vertical drainage pipe 90 for blocking debris from flowing into an accommodation space 902 of the vertical drainage pipe 90. The vertical drainage pipe 90 includes a porous surrounding wall 901 inserted into the soil (not shown) for irrigation or for water infiltration into the soil. Water collected in the accommodation space 902 may permeate through the porous surrounding wall 901 to soak the soil. In this case, the soil at different levels may not be soaked evenly. Especially, when the rainfall amount/frequency is relatively low, the soil near the ground level may become dry for sustained failure to retain sufficient water/moisture thereby rending the land lot unusable for agriculture.